Southward [of Nan Dungortheb and Dor Dínen] lay the guarded woods of Doriath, abode of Thingol the Hidden King, into whose realm none passed save by his will. Its northern and lesser part, the Forest of Neldoreth, was bounded east and south by the dark river Esgalduin, which bent westward in the midst of the land; and between Aros and Esgalduin lay the denser and greater woods of Region. Upon the southern bank of Esgalduin, where it turned westward towards Sirion, were the Caves of Menegroth; and all Doriath lay east of Sirion save for a narrow region of woodland between the meeting of Teiglin and Sirion and the Meres of Twilight. By the people of Doriath this wood was called Nivrim, the West March; great oak-trees grew there, and it also was encompassed within the Girdle of Melian, that so some portion of Sirion which she loved in reverence of Ulmo should be wholly under the power of Thingol.

The Silmarillion, Quenta Silmarillion, Ch 14, Of Beleriand and its Realms

And Melian was [Thingol's] Queen, wiser than any child of Middle-earth; and their hidden halls were in Menegroth, the Thousand Caves, in Doriath.

The Silmarillion, Quenta Silmarillion, Ch 4, Of Thingol and Melian

[Thingol] withdrew all his people that his summons could reach within the fastness of Neldoreth and Region, and Melian put forth her power and fenced all that dominion round about with an unseen wall of shadow and bewilderment: the Girdle of Melian, that none thereafter could pass against her will or the will of King Thingol, unless one should come with a power greater than that of Melian the Maia. And this inner land, which was long named Eglador, was after called Doriath, the guarded kingdom, Land of the Girdle. Within it there was yet a watchful peace; but without there was peril and great fear, and the servants of Morgoth roamed at will....

The Silmarillion, Quenta Silmarillion, Ch 10, Of the Sindar

[By] the Girdle of Melian was Doriath defended through long ages from the evils without. But now Thingol lay dead,... and with his death a change came also upon Melian. Thus it came to pass that her power was withdrawn in that time from the forests of Neldoreth and Region, and Esgalduin the enchanted river spoke with a different voice, and Doriath lay open to its enemies.